1. Field of the Invention
This invention relates to a spread spectrum communication system, and its receiver and transmitter, and more particularly, is suitable to a portable telephone device, etc. of a spread spectrum communication method.
2. Description of the Related Art
Heretofore, in a spread spectrum communication system, the Pseudo Noise code (PN code) transmitted from a transmitter is detected by a receiver, so that the PN code used for the spread spectrum is synchronous between the transmitter and the receiver.
Here, such a spread spectrum communication system will be described with reference to FIG. 1.
As shown in FIG. 1, in a transmitter 2 of the spread spectrum communication system 1, information data S1 as a transmission data is inputted to an information modulating part 3. The information modulating part 3 performs the primary modulation such as the frequency modulation or the phase modulation on a predetermined carrier wave by using the information data S1. The obtained transmitting signal S2 is outputted to a multiplier 4. PN code S3 generated at a PN code generator 5 has been inputted in the multiplier 4, and the multiplier 4 multiplies the transmitting signal S2 by the PN code S3 so as to spread the spectrum of the transmitting signal S2.
The transmitting signal S4 which is a spread spectrum signal is inputted to a frequency converting part 6 where it is frequency-converted into a signal having a high frequency, and then it is amplified by a radio frequency (RF) amplifying part 7 to be irradiated through an antenna 8 as a transmitting signal S5.
Meanwhile, in a receiver 9, the transmitting signal S5 transmitted from the transmitter 2 is received at an antenna 10, and is inputted to a RF amplifying part 11. The RF amplifying part 11 amplifies the receiving signal to output it to a frequency converting part 12. The frequency converting part 12 frequency-converts the receiving signal of a high frequency into the receiving signal S6 of a low frequency to output it to a multiplier 13 and a PN detecting part 14.
The PN detecting part 14 detects the PN code from the receiving signal S6, and outputs timing signal S7 for initializing the PN code to a PN generator 15. The PN generator 15 generates PN code S8 at a proper timing based on the timing signal S7 to output it to the multiplier 13. The PN code S8 is multiplied by the receiving signal S6 by the multiplier 13, so that the spread spectrum receiving signal S6 is inverse-spread. The inverse-spread receiving signal S9 is inputted to an information demodulating part 16 where it is demodulated by performing the reverse processing to that done in the information modulating part 3 at a transmission side and is outputted as an information data S10.
In the spread spectrum communication system 1, generally, if the spread rate is made high, the jamming-resistance is improved, and it makes it possible to communicate surely even when a carrier-to-noise ratio C/N is low. However, if the spread ratio is made high, the length of PN code used in PN code detection at a reception side (referred to as detection code length. Note that the detection code length does not always correspond to the period of PN code.) becomes long.
However, if the detection code length is lengthened, the construction of the matched filter becomes complicated as in the case of using a matched filter as the PN detecting part 14, and there occurs the necessity of a standard oscillator of high accuracy. On the contrary, if the detection code length is shortened, the PN code can not be detected at a reception side when the carrier-to-noise ratio C/N is low, and there occurs the problem that the detection accuracy (i.e., reliability) becomes low.
As a method for solving the problems, there is a method for realizing the improvement of detection accuracy, the simplification of circuit construction, and the relief of frequency accuracy, by transmitting repeatedly the comparatively short PN code before the data transmission, and detecting the PN code several times at a reception side.
Here, as shown in FIG. 2, the case where the PN code is transmitted five times repeatedly before the data transmission will be explained based on the above method. In this case, the opportunities of detecting PN code exists five times at a reception side, and it is assumed that the synchronization of PN code can be detected if two detections can be performed among five opportunities of detection. As a case corresponding to this condition, there are two cases for example, one is the case where first and second PN codes are detected and the rest of the codes are not detected, and other is the case where second and third PN codes are detected and the rest of the codes are not detected.
In both cases, since the PN detecting part can not know where does the detected PN code ranks, it can not be specified how far the head of the data is apart from the start point of the detection. Therefore, the start code representing the head of the data is needed to be inputted in the data in this method. In this way, this method can solve the above problem, but the redundant code which is the start code is needed to be added and there still have a insufficient point as a solution.